There are various motivations for retconning. The changes may occur to accommodate sequels or derivative works, allowing newer authors or creators to revise the diegetic (in-story) history to include a course of events that would not have been possible in the story's original continuity. Retcons allow for authors to reintroduce popular characters and resolve errors in chronology.

The first published use of the phrase "retroactive continuity" is found in Elgin Frank Tupper's 1974 book The theology of Wolfhart Pannenberg:[4]

"Pannenberg's conception of retroactive continuity ultimately means that history flows fundamentally from the future into the past, that the future is not basically a product of the past."

The first known printed use of "retroactive continuity" referring to the altering of history within a fictional work is in All-Star Squadron #18 (February 1983) from DC Comics. The series was set on DC's Earth-Two, an alternate universe in which Golden Age comic characters age in real time. All-Star Squadron in particular, was set during World War II on Earth-Two; as it was in the past of an alternate universe, all its events had repercussions on the contemporary continuity of the DC multiverse. Each issue changed the history of the fictional world in which it was set. In the letters column, a reader remarked that the comic "must make you [the creators] feel at times as if you're painting yourself into a corner," and, "Your matching of Golden Age comics history with new plotlines has been an artistic (and I hope financial!) success." Writer Roy Thomas responded, "we like to think that an enthusiastic ALL-STAR booster at one of Adam Malin's Creation Conventions in San Diego came up with the best name for it a few months back: 'Retroactive Continuity'. Has kind of a ring to it, don't you think?"[5] The term then took firm root in the consciousness of fans of American superhero comics.

At some point, "retroactive continuity" was shortened to "retcon," reportedly by Damian Cugley in 1988 on Usenet. Hard evidence of Cugley's abbreviation has yet to surface, though in a Usenet posting on August 18, 1990, Cugley posted a reply in which he identified himself as "The originator of the word 'retcon.'"[6] Cugley used the neologism to describe a development in the comic book Saga of the Swamp Thing, which reinterprets the events of the title character's origin by revealing facts that previously were not part of the narrative and were not intended by earlier writers.

Some retcons do not contradict previously established facts but instead fill in missing background details, usually to support current plot points. Thomas referred to "retroactive continuity" in this sense, as a purely additive process that did not undo any previous work; such additions were common in All-Star Squadron. Kurt Busiek took a similar approach with Untold Tales of Spider-Man, a series which told stories that specifically fit between issues of the original The Amazing Spider-Man series, sometimes explaining discontinuities between those earlier stories. John Byrne utilized a similar structure with X-Men: The Hidden Years. Possibly the earliest Marvel Comics example of new stories placed in between long-established stories was the 1977-8 magazine The Rampaging Hulk. In The Godfather Part II, the character Frank Pentangeli is introduced as an old friend of the family though he is not referenced in the first movie; similarly Don Altobello is one of the "old time" Dons, though he is not mentioned until The Godfather Part III. Neither addition affects the plot line of the previous films.

A similar concept is that of secret history, in which the events of a story occur within the bounds of already-established events (especially real-world ones), revealing different interpretations of the events. Some of Tim Powers's novels use secret history, such as Last Call, which suggests that Bugsy Siegel's actions were due to his being a modern-day Fisher King.

Alan Moore's additional information about the Swamp Thing's origins did not contradict or change any of the events depicted in the character's previous appearances, but instead changed the reader's interpretation of them. Such additions and reinterpretations are very common in Doctor Who.[3]

In the Stargate franchise, the television series that followed the original film established the main antagonist of the film, Ra, as part of an alien species called the Goa'uld, and included many more members of the species as enemies in the series, where the film had initially established Ra as the last of his kind.

Retcons often add information that contradicts previous information. This frequently takes the form of a character who was shown to have died, but is later revealed to have somehow survived. This is a common practice in horror films, which may end with the death of the monster who goes on to appear in one or more sequels. The technique is so common in superhero comics[3] that the term "comic book death" has been coined for it. An early example of this type of retcon is the return of Sherlock Holmes, whom writer Arthur Conan Doyle apparently killed off in "The Adventure of the Final Problem" in 1893,[7] only to bring him back, due in large part to readers' responses, with "The Memoirs of Sherlock Holmes" in 1894.[8]

In many of his detective novels, Rex Stout implies that his character Nero Wolfe was born in Montenegro, giving some details of his early life in the Balkans around World War I. But in 1939's Over My Dead Body, Wolfe tells an FBI agent that he was born in the United States. Stout revealed the reason for the change in a letter obtained by his authorized biographer, John McAleer: "In the original draft of Over My Dead Body Nero was a Montenegrin by birth, and it all fitted previous hints as to his background; but violent protests from The American Magazine, supported by Farrar & Rinehart, caused his cradle to be transported five thousand miles."[9]

In the 1940s and 1950s, Isaac Asimov placed the planet Trantor, capital of the Galactic Empire, at the "center of the galaxy," but later astronomical research indicated that the actual Galactic Center might be occupied by a supermassive black hole, making human life there impossible; in later works, Asimov adjusted his galaxy and Trantor's location in it.

When E.E. "Doc" Smith wrote the original The Skylark of Space, space flight was a completely theoretical proposition. However, the last book of the series, Skylark DuQuesne, was written in 1963, when the United States and the Soviet Union were involved in the space race. Smith adjusted accordingly the past of his series, mentioning an American base and a Soviet one being established on the Moon prior to the protagonist Seaton discovering faster-than-light flight.

The 1978 TV series Dallas annulled its entire Season 9 as being just the dream of another character, Pam Ewing. This was done in order to offer a supposedly plausible reason for the major character of Bobby Ewing, who had died onscreen at the end of Season 8, to be alive after actor Patrick Duffy wanted to return to the series. This season is sometimes referred to as the "Dream Season" and was referred to humorously in later TV series such as Family Guy.

The video game Saints Row introduced a nameless male protagonist whose appearance players could alter by visiting plastic surgery clinics. The final story mission ends with a yacht exploding, killing everyone aboard including the protagonist. However, in the sequel, it is revealed that the protagonist survived, and was in a coma for five years before receiving extensive plastic surgery during their recovery. Additionally, players now have the option of playing as a female protagonist, thus retconning the protagonist's previously established male gender. Despite being able to play as a female character, and despite explicitly having received plastic surgery to change his/her appearance, the characters from the first game perfectly recognize the protagonist when they meet again, simply saying things like "You look different, did you do something with your hair?".

Similarly, in the beginning of Saints Row: The Third, the hugely popular character Johnny Gat was killed. While his death happened off-screen, it was explicitly stated throughout the game that he had been killed. His zombie could even be unlocked as an ally upon completing the game. But his death was later retconned away (possibly in response to negative outcries from fans), as in Saints Row IV, it was revealed that Gat was in fact abducted by aliens. Also, in Saints Row IV's expansion, Gat out of Hell, in one of the five endings, the entire series is retconned and Johnny Gat, Kinzie Kensington, and Matt Miller appear as cops.

The 1998 American re-imagining of Godzilla ended on a cliffhanger in which one of Godzilla's eggs survived the bombing of Madison Square Garden, foreshadowing a sequel. While a sequel never happened, an animated series continued the story. However, this series retconned the movie's ending slightly. In the film, the remaining Godzilla egg hatched alone. Whereas in the animated series, the character Nick Tatopoulos (played by Mathew Broderick in the film) returns to the crash site with the military and accidentally discovers the egg before it hatches, upon which the new Godzilla imprints on him. Other minor changes included Kevin Dunn's character from the film being demoted from Colonel to Major without explanation. In 2004, the character was retconned when Toho, the character's owners, officially renamed the 1998 creature from "Godzilla" to Zilla[10] and has since become a part of Toho's Godzilla franchise.[11][12]

During the first season of My Little Pony: Friendship is Magic, a flashback reveals that the character Pinkie Pie used to live on a rock farm with her parents and two sisters, and that Pinkie Pie earned her cutie-marks during a party she threw for the four of them. However, the fourth season introduced a third sister named Maud Pie. To account for this new character and her apparent absence from the previous flashback, the party Pinkie threw for her family was retconned to include Maud.[citation needed]

Unpopular or embarrassing stories are sometimes later ignored by publishers, and effectively erased from a series's continuity. Later stories may contradict the previous ones or explicitly establish that they never happened. An unpopular retcon may itself be retconned away, as happened with John Byrne's Spider-Man: Chapter One.

It is believed by some readers[who?] that Ian Fleming's The Spy Who Loved Me was subtracted from the James Bond novel continuity.[citation needed] The book received mostly bad reviews from critics and Fleming himself didn't like it, going so far as to request there be no paperbacks or reprints and sold the film rights to Eon Productions under the condition that no part of the book be adapted.[13] When Fleming wrote the next novel in the series, On Her Majesty's Secret Service, it is revealed that the organization SPECTRE had not been heard of since the events of Thunderball. This ignores the events of The Spy Who Loved Me, which had James Bond hot on the trail of SPECTRE. However, it is also true this is said in the first chapter of the novel, which takes place before The Spy Who Loved Me, while the book takes place after Spy only from chapter 6 onwards.

In the second life of the sitcomMama's Family, characters Buzz and Sonja were briefly mentioned in the first episode of the third season but then were never spoken of again, as if they never existed; especially when Vint, their father, and Naomi were planning on having a baby.

Retroactive continuity is similar to, but not the same as, plot inconsistencies introduced accidentally or through lack of concern for continuity; retconning is done deliberately. For example, the ongoing continuity contradictions on episodic TV series such as The Simpsons (in which the timeline of the family's history must be continually shifted forward to explain them not getting any older. For instance, when the series started, Bart would have to have been born in about 1980, but that would make him 35 years old as of 2015) reflects intentionally lost continuity, not genuine retcons. However, in series with generally tight continuity, retcons are sometimes created after the fact to explain continuity errors, such was the case in The Flintstones, where Wilma Flintstone was mistakenly given two separate maiden names, "Pebble" and "Slaghoople", over the course of the series; upon discovering the discrepancy, the producers settled on "Slaghoople" and retconned it into later series in the franchise.

Retconning is also generally distinct from replacing the actor who plays a part in an ongoing series, which is more commonly an example of loose continuity rather than retroactively changing past continuity. The different appearance of the character is either ignored (as was done with the character of Darrin Stephens on the television series Bewitched or Vivian on The Fresh Prince of Bel-Air), or explained within the series, such as with "regeneration" in Doctor Who, or the Oracle in The Matrix Revolutions. Sometimes, there are referential, inside jokes on actor changes in the show, such as the case with My Wife & Kids and Roseanne, where a daughter (Claire Kyle and Becky Conner respectively) was played by 2 actresses.

It also differs from direct revision. For example, when George Lucas re-edited the original Star Wars trilogy, he made changes directly to the source material, rather than introducing new source material that contradicted the contents of previous material.

Retconning is not the same as a reboot or reimagining which completely discards the original timeline, such as in Battlestar Galactica. However, there have been partial reboots of franchises where the core of the franchise is still canonical but the expanded universe is relegated to a secondary continuity which, while not completely invalid, is subject to revision and critical review. Robotech is an example of this. With the release of the 2006 sequel film Robotech The Shadow Chronicles, Harmony Gold established that only the original 1985 animated series and the 2006 sequel film are considered canonical relegating the aborted Robotech II: The Sentinels, comics, and novels from the 80s and 90s to secondary continuity and, if elements are used from them, they are subject to selective revision and updating as appropriate to mesh with future canonical productions and to prevent conflict with the original animated series. While the Jack McKinneyRobtotech novel End of the Circle is evidently no longer canon, the prequel comic Robotech: Prelude to the Shadow Chronicles establishes that the general storyline of The Sentinels still occurred in some fashion but various elements, including the timeline, specific unfolding of events, and some characterizations are different from what was previously depicted in earlier comics and novels. In such cases, the franchise producer may state that there is no intention to address the changes through remakes or direct retellings of such stories. It is essentially left to the viewer's imagination as to how differently the revised story unfolded.